In the medical field, needlestick is a continuing problem. Needlestick is the term used when a contaminated needle pricks or cuts a person. A contaminated needle can readily become a source of infection. Another problem with syringes is with sharing of needles which can also result in infection.
For this reason, it is known to provide a syringe having a needle that has a particular design such that the needle can be used only once. One way that this is achieved is to retract the needle back into the syringe body after use. This also provides the other advantage of minimising needlestick injury.
Many types of single use syringes are known, and most have some form of retraction mechanism (also called a shoot back mechanism) to shoot the contaminated needle back into the syringe body after use.
It is known to provide single use syringes where the needle is spring biased and is held in place by a cuttable member. In order to ensure that the member is properly cut thereby releasing the needle, a two-part cutting action is usually required. In the two-part cutting action, the plunger typically has a forward cutting edge and the front of the needle also has some form of cutting edge and a double cutting action is required to ensure that the cuttable member is property cut to release the needle. It is found that attempting to cut a cuttable member to release a spring-biased needle is not always reliable and does not always work.
Another type of known single use syringe has a base member that grips an enlargement on the needle. The plunger pushes the base member forwardly, which further compresses a spring that is around the needle. The pushing action releases the grip between the base member and the enlargement on the needle. This allows the needle containing the enlargement to be shot back into the plunger while leaving the spring behind. The spring is initially compressed partially but then becomes compressed fully as the plunger forces the base member forwardly. This arrangement requires a needle having a special design (containing an enlargement), which means that conventional needles cannot be used.
Another type of syringe has a plunger seal which moves on the plunger and where the plunger seal slides rearwardly along a bearing surface in response to a force being applied to the seal that is in excess of the operational force of the syringe. This arrangement begins the retraction sequence of the needle. With this arrangement, a specially designed plunger seal is required which moves relative to the plunger. Conventional plungers have a plunger seal that is fixed to the plunger.
Another type of syringe has a shoot back arrangement consisting of two parts which slide relative to each other to release the needle. The two parts consist of an outer part and an inner part. The inner part holds the needle and is biased by a helical spring. The two parts are held together by frictional engagement to each other. A plunger progressively pushes the outer part forwardly thereby progressively reducing the amount of frictional engagement between the outer part and the inner part until such time that the bias of the spring is sufficient to shoot the inner part away from frictional engagement with the outer part. This arrangement requires a sliding frictional grip to hold the two-part together which is considered quite risky and requires careful manufacture.
Another arrangement uses a needle holder having an elongate body portion in front and a head end in back and providing a spring under the head end that circumscribes the needle holder. The use of an elongate body in a needle holder makes this arrangement difficult for use in small and confined spaces.
It is found that retraction (shoot back) mechanisms that use a cutting action are not particularly reliable especially for small syringes, and there would therefore be an advantage if a retraction mechanism could be used that did not require a cutting action. While not wishing to be bound by theory, the small syringes have a very small needle holding attachment, and trying to cut part of the attachment away using a knife edge on the piston is not particularly reliable. For instance, variation in the plastic used in the attachment can cause the plastic to become too rubbery, or too hard, and this can make the cutting action incomplete or unreliable.
A useful type of retraction mechanism enables the needle holder to be retracted or shot back into the hollow plunger in a relatively straightforward manner. There is a disadvantage if the retraction is impeded. It is always necessary to have some form of seal or cover over the front of the plunger and this seal or cover needs to be broken or pierced to allow the needle holder (containing the contaminated needle) to be shot back into the hollow plunger and through or past the seal or cover. It is found advantageous if the front of the plunger contains a relatively thin seal as this can improve the reliability of the shoot back mechanism. Conversely, it is found to be a disadvantage if the front of the plunger contains a stopper, a plug, or some other relatively bulky member. If a stopper or plug is used, there is no way that the needle holder can pierce through the stopper or plug, and therefore, it is necessary for the needle holder to be shot back with sufficient force to also push back the stopper or plug into the plunger body. Therefore, there would be an advantage to provide a single use syringe with a retraction mechanism that has the front of the plunger containing a relatively thin seal through which the needle holder can pass as opposed to a relatively bulky stopper or plug.
Another important consideration with single use retractable needle syringes is the force required to activate, or to trigger, the shoot back mechanism. This force is the pressure that needs to be applied to the rear of the plunger to push the plunger hard up against the needle holder to trigger release of the needle holder. Typically, this force will be applied by the user's thumb. It is found that this force can be up to 9 kg to enable a conventional shoot back needle to be triggered, and this is found to be excessive. However, simply making the shoot back mechanism more “flimsy” is not a solution, as this can result in accidental or inadvertent triggering of the shoot back mechanism. Therefore, there would be an advantage if a single use retractable needle syringe could be manufactured that would be reliable but that would also reduce the amount of force required to trigger the shoot back mechanism.
Attempts have been made to reduce the triggering force. However, previous attempts have been directed to providing a complex design to the front of the plunger. The complex design to the front of the plunger includes having a part of the plunger front face projecting forwardly from the remainder of the plunger front face. Thus, only part of the plunger (the forwardly projecting part) initially contacts the needle holder to trigger the shoot back mechanism. This type of arrangement prevents the front of the plunger from having a “conventional” design which is usually a substantially flat or planar seal extending over the otherwise open front of the plunger. As mentioned previously, there is an advantage in having a substantially “conventional” plunger head to enable the needle to be shot back through the plunger head and into the plunger body without needing to push back a stopper etc. Also, having a relatively ordinary plunger head allows the needle to shoot back through the plunger head without carrying any significant debris (from the plunger head) which can impede efficient retraction of the needle. Another advantage with having a relatively conventional plunger head design, is that the medical practitioner can more easily see the operation of the plunger and especially how close the plunger is to the end of its stroke. With a complicated plunger head design, it becomes difficult to see where the “end” of the plunger actually is with respect to the front of the syringe.
Another disadvantage with many existing single use syringes is that the mechanism required to shoot back the needle often has a portion protruding into the syringe barrel, and can result in incomplete expulsion of the liquid from the barrel and through the needle. In other words, many existing single use syringes have a “dead space” where the syringe fluid can collect and where the fluid will not be properly expelled from the syringe. This can result in incomplete or erroneous levels of fluid being injected into a person. There is therefore a significant advantage if the volume of “dead space” could be reduced, prior to triggering the shoot back mechanism. As mentioned previously, it is also advantageous to reduce the force required to trigger the shoot back mechanism. The mechanism described above to reduce the force to trigger the shoot back mechanism (the complex plunger design) results in part of the plunger front face projecting forwardly with respect to the remainder of the plunger front face. This particular design can also increase the “dead space” due to the design of the plunger.
Therefore, there would be an advantage to provide a design that can reduce the force to trigger the shoot back mechanism but which can also reduce the dead space, and which is reliable in use. The applicant considers that reliability can be provided by having a shoot back mechanism where the needle shoots through a relatively thin portion of the plunger head (therefore there is no need to push back any stopper, and there is no “shatter plate” and the like which can carry debris back into the plunger body), and where the plunger head is otherwise of a conventional type shape which is found to be best in reducing dead space and is also relatively easy to manufacture.
While there is an advantage in having a “conventional” relatively flat type plunger head to reduce dead space, this conventional shape can sometimes not be as effective as possible in reducing dead space because of the particular construction of the “reduced force” shoot back mechanism. Therefore, there would be an advantage to provide a particular construction of part of the shoot back mechanism which would reduce dead space but still allow a “conventional” type plunger head to be used.
Another disadvantage with some of the spring biased shoot back needles, is that in practice, it is possible to reassemble the mechanism which means that the syringe and needle can be reused. Clearly, a single use syringe should be manufactured in such a manner that it would be impossible to reuse the syringe. Therefore, there would be an advantage in having some form of “destructive mechanism” which would render the syringe unable to be reused.
It will be clearly understood that, if a prior art publication is referred to herein, this reference does not constitute an admission that the publication forms part of the common general knowledge in the art.